1. Field of the Invention
The present invention relates generally to the determination of physiological functions and parameters and more specifically to systems and methods for non-invasively measuring left ventricular pressures of the blood within a left ventricle of a heart.
2. Description of Related Art
The function of the heart is well understood due in large part to the advanced measurement techniques available to cardiologists and other medical professionals. Many parameters of the cardiac cycle may be measured and monitored to determine the general well being of a patient. Some of the monitoring and measurement techniques are invasive, requiring sensors or other measurement devices to be placed within the heart or vessels connected to the heart, while other techniques are non-invasive.
Some of the most important parameters associated with the cardiac cycle are pressures within the left ventricle of the heart. The left ventricle is the chamber of the heart responsible for pumping oxygenated blood throughout the body. The walls of the left ventricle act much like a spring: the more the walls are stretched, the more force the walls may impart to the blood when the ventricle contracts. For example, a normal ventricle is able to expel about 70 mL of blood with a single stroke after being stretched with a pressure (inside the ventricle) of about 10 mmHg. If the walls of the ventricle are under-stretched, the heart delivers less blood when the ventricle contracts. This may happen, for example, when a person has bled so much that the volume of blood left in the body is inadequate to properly fill the heart. The blood pressure then drops significantly since the heart cannot pump out a sufficient amount of blood. Another example is when a person experiences myocardial infarction. These “heart attacks” will sometimes cause a significant portion of the heart to die, and the remaining portion of the heart must compensate for the dead areas to keep the person alive. This often requires that the blood coming into the ventricle stretch the ventricle with more force than normal in order to get the normal amount of flow. In this particular instance, a pressure of 20 mmHg may be required to stretch the left ventricle enough in order to pump the same 70 mL of blood with each stroke.
Several heart-related conditions are directly linked to pressures of blood within the heart. When the heart requires an inordinate filling pressure to pump the same amount of blood that usually would take only 10 mmHg pressure, the patient is considered to have congestive heart failure (CHF). If the heart cannot pump an adequate amount of blood, the patient's blood pressure will fall. However, if the pressures in the left ventricle exceed a certain point, then the blood may back into the lungs, thereby forcing blood and other fluids into the airspaces of the lungs. Under these circumstances, the patient is literally drowning. This condition is commonly referred to as pulmonary edema.
A patient may also have problems that are not heart related that exhibit similar symptoms to those caused by pulmonary edema. For example, fluid in the lungs may also be caused by the kidneys holding excess water, the liver not functioning properly, or the lining of the lungs being damaged, as in the case of drowning victims. When patients are examined in the hospital, it is often impossible to just look at the patient and determine what the problem is.
Heart problems causing symptoms such as those described above may be more easily diagnosed if the filling pressures of the heart are known. When the filling pressures of the heart are high, the heart is likely at fault. If the filling pressures are low, then the heart is usually a victim of other problems. The filling pressure, also referred to as the left ventricular end-diastolic pressure (LVEDP), is the pressure of blood within the left ventricle of the heart immediately following diastole (i.e. the filling cycle of the left ventricle) and before systole (i.e. when the left ventricle contracts to pump blood through the body).
There are primarily two procedures that have been used by physicians to obtain information about pressures within the heart. Both are forms of catheterization, but one approaches the heart from the left side and the other from the right. In a typical “left-heart catheterization,” a needle is inserted in the groin, followed by a long flexible tube, the catheter, which is advanced to the heart through a series of blood vessels. The catheter finally passes into the large vessel known as the aorta on the downstream side of the left ventricle. The left-heart catheterization is performed in a specialized lab by a trained cardiologist, who uses special infusible dyes to make parts of the heart visible. A left-heart catheterization provides a means of directly measuring pressure and flow in the heart. This is also the technique used to outline the coronary arteries and determine whether there is a blockage in the arteries in need of a bypass or balloon angioplasty.
In a “right-heart catheterization,” a long flexible tube called a Swan-Ganz catheter, is inserted in the neck or under the clavicle and is advanced into the right side of the heart. The pressures that are most desired by physicians are those pressures within the left side of the heart, and blood leaving the right side of the heart must travel to the lungs before returning to the left side of the heart. The catheter therefore includes a balloon attached to its tip, and the balloon is inflated with a small amount of air to allow the catheter to float through the right atrium and right ventricle into the pulmonary artery. The catheter continues to move through the pulmonary artery until it wedges within a distal branch of the pulmonary artery that is smaller in diameter than the balloon. Since the balloon blocks blood flow through the small vessel in which it is wedged, a static column of blood develops between the tip of the catheter and the left atrium. In theory, the pressure measured within this static column of blood upstream of the lungs (i.e. the “wedge pressure”) approximates the left atrial pressure. If no obstruction exists between the left atrium and the left ventricle, the wedge pressure may also be a good approximation for the LVEDP. Most of the time, these pressure approximations are accurate enough to allow decisions to be made in the management of critically ill patients.
Left-heart catheterization requires a patient to be very stable, while right-heart catheterization may be performed on critically ill patients. Right-heart catheterization is typically performed in the intensive care unit (ICU), the operating room, or in the emergency room, and is conducted by a specially trained physician. The procedure further requires constant monitoring of heart and lung function, and the catheter may only remain in place for one or two days due to the risk of infection and trauma to the heart. The placement of the Swan-Ganz catheter alone may cost up to two thousand dollars.
Although the information provided by Swan-Ganz catheters is desperately needed, the use of these catheters has fallen out of favor with physicians during the last five years. The procedures are performed much less frequently due to recent studies that suggest the catheters may contribute to complications in patients. Even taking into account the serious illness of patients requiring a Swan-Ganz catheter, the act of using the catheter seems to increase the morbidity and mortality of patients. However, these procedures are still performed when there is no other choice.
A need therefore exists for a system and method that would allow physicians to quickly and accurately determine the condition of a patient's heart without subjecting the patient to the dangers and risks of catheterization. A system is further needed that would allow the health of a patient's heart to be determined in a non-invasive manner that does not require physical penetration of the patient's skin or blood vessels. More specifically, a need exists for a system and method that would inexpensively allow determination of left ventricular pressures within the heart using non-invasive measuring techniques.